Workmen engaged in brake maintenance and repair operations are frequently exposed, in the workplace, to aerosols containing high concentrations of chrysotile asbestos fiber, in excess of the OSHA legislated threshold limit value. This study will undertake the determination of an asbestos-exposure index, on both the light and electron microscopic level, utilizing lung parenchyma. Lung parenchyma will be studied from: 25 brake workers; 25 insulation workers (a positive control); 25 asbestos factory workers (a positive control); 25 white collar workers, with no known asbestos exposure (a negative control). Study methodology will include light microscopic analysis of 175 micron thick ashed tissue stacks, and a detailed transmission electron microscopic study of tissues obtained from the same block. Asbestos bodies, uncoated asbestos fibers (their amounts and varieties) and other particles, will be quantified. Simultaneously, specially stained sections from the same tissue blocks will be studied for pathology. Each tissue will be studied to determine the presence or absence of lung disease, especially lung scarring. Slides will be stained with hemotoxylin eosin, Van Gieson, PAS, and Gordon and Silver stain for the determination of scar tissue development. A correlation will be attempted between the presence of inorganic particles and a state of lung pathology. On the basis of a quantitative comparison with asbestos fiber content in both positive and negative control groups an asbestos exposure index may be determined for the brake workers.